The removal of relatively fine particulates from a gas stream, using a scrubbing fluid, together with the subsequent separation of the gas and the scrubbing fluid, is well known, and is often carried out by means of a so-called wet scrubbing process.
More particularly, the removal of relatively fine particulates from a gas stream, using a scrubbing fluid, and the subsequent separation of the gas and the scrubbing fluid, is applied in the treatment of the hot off-gases from a Sinter Process such as that which forms part of many of the modern iron making processes.
The off-gases from the Sinter Process typically have a temperature of around 150° C., with a short duration maximum of around 180° C. to 200° C. The gases contain the products of a carbon fuelled combustion process with a relatively large amount of excess air. The gases also contain dust, products of incomplete combustion (including dibenzo-furans, PCB's and related compounds), acid gases (derived from sulphur and other impurities in the feed stocks) and condensed fumes. These fumes typically contain condensed alkali and other metal salts (usually chlorides) and condensed silica compounds with other similarly sized fine particulates resulting from decrepitation and other processes that occur within the sintering process.
As a result of the processes that occur, the total dust load is essentially made up of two distinctly sized groups, a relatively coarse fraction and a relatively fine fraction. The coarse fraction is usually extracted from the off gases using cyclonic or other equivalent separators and this is usually done between the sinter process and the main extraction fans which are used to draw the combustion air through the sinter process. Removal of this coarse dust upstream of these fans ensures minimum wear on these fans.
Downstream of the fans, the fine dust and other contaminants have to be removed before the off-gases can be discharged to atmosphere. Current technologies for this utilise bag filters, electrostatic precipitators and wet electrostatic precipitators.
In many instances, however, the proportion of alkali salts (potassium and sodium salts) causes the dust that is to be removed, unsuitable for bag filters and normal electrostatic precipitators, leaving wet electrostatic precipitators as the only existing technology option which is capable of meeting the current requirements regarding final dust concentrations.
Normal wet scrubbing processes and related systems are typically able to remove particles at relatively high efficiencies down to particle sizes of around 3 to 5 microns. A disadvantage of these wet scrubbing processes and systems is however their inability to achieve removal efficiencies of above 90% of particle sizes of less than 0.05 micron. A further disadvantage is the relative large bulk of the known wet scrubbing systems. Another disadvantage of the known wet scrubbing systems is the relatively large floor area required by those systems that are capable of achieving removal efficiencies of above 90% of particle sizes of less than 0.05 micron, such as the conventional Electrostatic Precipitator (“ESP”) or the bag house installation.
An additional disadvantage of the typical primary equipment, or components of the assemblies, or so-called packs of components used in the known wet scrubbing systems is the relative difficulty with which they are moulded or cast, using low cost plastics, resins and reinforced plastics or resins (with or without abrasion inhibiting fillers). A further disadvantage of the equipment and components is the relative difficulty with which they are assembled and maintained, typically requiring the use of specialist tools and/or support services.
The influence of the degree of mixing, and hence the contact accomplished, during the multi-phase interaction in the scrubbing process on the efficiencies obtained with a gas scrubbing process and the associated equipment is also well known. The use of equipment for intensifying the mixing and contacting during the multi-phase interaction is therefore common practice.
High intensity mixing and contacting is for example accomplished in the so-called Multiphase Staged Passive Reactor (“MSPR”), with its smoothly contoured design, substantially as described in U.S. Pat. No. 5,741,466 and French Patent No 1.461.788.
The MSPR, as described in the above French Patent, is a static, co-current contacting device for contacting a flow of gas with a typically smaller volumetric flow of liquid, mixture of liquids or slurry. The device is typically used for the purposes of enhancing mass and/or heat transfer in, the removal of fine particulates from, and the creation and dispersion of fine liquid or slurry droplets into a gas stream. The mass transfer typically includes evaporation or partial evaporation of the liquid, the partial or complete condensation, dissolution or reaction of gaseous or vapour components within the gas onto, into or with the liquid(s) or slurry, or the partial or complete removal of a component within the liquid, mixture of liquids or slurry into the gas stream.
The MSPR, as described in the above U.S. patent, has no moving parts, and is typically used for producing interphasic interaction of a first substance in a liquid phase with a second substance in a non-miscible liquid phase, a solid phase or a gaseous phase, wherein the phases of the first and the second substances respectively are characterised by different relative densities. This MSRP typically comprises a plurality of stages defining a flow path for the first and the second substances, each stage being shaped to define a substantially curved flow path having a centre of curvature located to one side of the flow path, and wherein adjacent stages have a respective centre of curvature on opposite sides of the flow path whereby, as the substances flow through the reactor, particles of the second substance are forced to migrate through the first substance, first in one direction and then in substantially in the opposite direction to promote interphasic interaction.
The MSPR has characteristically a relatively smoothly profiled and constant annular flow passage, so that when applied in gas scrubbing, the scrubbing fluid that collides with the wall of the profile tends to accumulate on the inside curve of each bend in the profile and then “drips off” as a semi continuous flow of droplets. This flow of droplets pulls away from the accumulated layer of fluid as a result of induced turbulence from the gas as it flows around the inside of the bend and centrifugal forces resulting from the velocity of the fluid over the surface of the flow passage. In general, not all of the scrubbing fluid will come off the surface of the profile, leaving a significant proportion to flow over the subsequent surface. As a result, this part of the scrubbing fluid will not present itself to the bulk of the dust in the gas flow. Also, for a given gas velocity, the droplets that do leave will be relatively large droplets, all of which do not leave from the same point on the inside radius. Some droplets also tend to be released within the shadow of a droplet, that was released a few millimeters earlier, rather than to fill the gaps between previously and/or simultaneously released droplets. As a result, a relatively low proportion of the total gas flow will be traversed by the droplets that are released, than would be the case if the same number of drops were released uniformly around the perimeter.
In addition, much of the scrubbing fluid that is released will be released from relatively far around the inside of the bend. On the inside of the bend, because of turbulence within the gas on the inside of the bend, the shear forces from the high velocity gas will not all be in the direction of the bulk flow. As a result, there will be a reduced velocity input from the gas into the surface layer of the scrubbing fluid on this part of the surface of the flow profile. This, together with viscous drag from the stationary wall of the flow profile within the film of scrubbing fluid, will cause the film velocity at the release of the droplet to be significantly lower than that of the film velocity upstream of the inside radius.
This reduced velocity and the orientation of this velocity with regard to the subsequent flow profile, results in specific disadvantages, including a relatively smaller number of larger droplets, thereby presenting a substantially reduced droplet surface area; and lack of penetration in that the majority of the droplets do not penetrate relatively far into the gas flow before the following bend causes them to move back towards the wall again as a result of both centrifugal action and inertia.
The resultant substantially reduced droplet surface area causes a relatively poor scrubbing efficiency per unit volume of scrubbing fluid that is released from the surface of the flow profile.
The lack of penetration causes a tendency for scrubbing fluid on one side of the flow profile to scrub the gas on that side of the profile only and for the fluid on the other side to scrub the gas on the other side only, with relatively little intermixing of the two flows of scrubbing fluid.
An additional disadvantage of partial contact of the gas with the scrubbing fluid, is that each time the scrubbing fluid leaves the solid surface of the walls of the flow profile, the gas flow tends to accelerate the droplets up to the gas velocity in that area, causing much of this additional velocity energy to be lost when the droplets re-combine with the film of fluid on the wall. The energy loss per unit of dust or fine droplet removal, per unit of gas scrubbing, becomes particularly significant when the droplets do not contact much of the total gas flow.
A related disadvantage of the MSPR is therefore its inability to retain the relative velocities of the gas and fluid flow through the flow profile, allowing the decline in relative velocities to reduce the ability of the scrubbing fluid droplets to remove fine dust and other particulates.
A further disadvantage of the MSPR is the lack of overall wear and chemical resistance of the material used for the manufacture of the MSPR as well as its ability to with stand environments of high impact, abrasion, corrosion and temperature demands such as those present with the scrubbing of the hot off-gases from the Sinter and other furnace related processes.